In studies of the structure of the receptors for interferon-alpha (IFN-alpha) we have utilized covalent cross-linking methods and a ligand blotting technique, which we recently developed. The ligand blot is analogous to a Western blot, but it utilizes a radiolabeled ligand instead of an antibody. This method may prove generally useful for the study of other receptors for which an antibody is not yet available. Using these techniques, we have found that 125I-rIFN-alpha A can bind specifically to two proteins with the apparent MW of 105 and 90 KDa. Studies using intact cells have demonstrated preferential down regulation of the 105 KDa receptor. Covalent cross-linking studies have also revealed the existence of a higher MW complex of approximately 240 KDa, which may represent the specific association of the IFN-binding protein with an intracellular protein. These studies reveal a greater complexity to the IFN-alpha receptors than was previously recognized. The relationship between these IFN-receptor complexes and responses to IFN are being examined. In studies to elucidate signal transduction mechanisms in the action of IFN-alpha, we have utilized protein kinase inhibitors. We have found evidence that a functional protein kinase C is required for induction of 2-5A synthetase mRNA and activity by IFN- alpha. However, protein kinase C activation alone is not sufficient for induction of 2-5A synthetase, suggesting that multiple signal transduction pathways may be required in the action of IFN. In studies of the IFNs as modulators of the expression of cellular proteins, we have found that rIFN-gamma treatment of myelomonocytic cell lines and peripheral blood monocytes, but not T lymphocytes, decreases expression of CD4, the receptor for HIV. Since monocytes/macrophages are considered to be an important reservoir for HIV in vivo, the effect of IFN-gamma in decreasing CD4 on these cells may have clinical relevance for the treatment of AIDS.